In modern motor vehicles, a multiplicity of electrical or electronic devices comprising inductive loads, for example valves, electric motors or relays, are actuated in a pulse-width-modulated manner by means of driver stages, usually switching transistors. PWM control can therefore be used, for example, to operate an electric motor of a starter for the purpose of starting an internal combustion engine or for pressure build-up in a brake installation. In conventional starting methods, a comparatively high switch-on current has to be provided by the vehicle battery, which can lead to a reduction in the terminal voltage of the vehicle battery.
The generic DE 10 2010 063 744 A1, which is incorporated by reference, solves this problem by virtue of a current limiting resistor being connected in series with the inductive load for the purpose of limiting the current during the starting phase by means of a FET transistor, with the result that a base current flows when the FET transistor is in the conductive state. A series circuit composed of a further PWM-controlled FET transistor and a further resistor is connected in parallel with the series circuit composed of the FET transistor and the current limiting resistor. The PWM control of said further FET transistor is used to control the current through the load, wherein, in the conductive state of said further FET transistor, the path with the current limiting resistor is shorted and the maximum current through the second resistor is limited. A disadvantage of this procedure is the high power loss generated during operation of the inductive load as a result of the resistors provided for limiting the current.
This disadvantage is overcome by the generic DE 10 2014 208 066.5, which is incorporated by reference, which has not been published as of the effective date of the present application, which describes a method for pulse-width-modulated current control of a load current of an inductive load, in which a current actual value is determined, a control difference from the current actual value and a current setpoint value is determined, the control difference is identified as an input value of a control algorithm for calculating a duty factor of the pulse-width modulation, and a power switch connected in series with the inductive load is actuated at the identified duty factor. The inductive load is transferred into an operating mode by means of a starting mode in that, for the purpose of carrying out the starting mode, the control algorithm identifies the current setpoint values, associated duty factors and a current target value based on a ramp function and controls the load current up to the current target value in accordance with the ramp function, and, for the purpose of carrying out the operating mode following the starting mode, the control algorithm identifies the duty factors based on an upper and a lower current setpoint value in such a way that, proceeding from the current target value, the load current is controlled between the upper and lower current setpoint value. However, this procedure cannot be applied to all applications on account of the ramp-shaped increase and the time delay associated therewith.